Heck, if you have the ability and access to the right type of equipment, you could easily do a running prototype via 3d printing. Nylon Carbon Fiber printing is chemical resistant and heat resistant up to about 300 degrees. The case halves won't see the kind of heat a head and cylinder do so it would probably survive fine for a while. Get your files right for the print and they could easily be scale adjusted to account for casting shrinkage.
Then you could do a 3d wax print and try your hand at lost-wax casting.